Thanks for your thorough research, I finally got it! And you're right, after the "trust you" scene is where it gets really obscure. I finally got it after finding Karlach and promising to help her; talking to Scratch only triggered a brief scene where Gale reminded me that he really, really needs to snack on a magic item really, really soon. Definitely something to test when the next patch comes out, to see if they cleaned it up a bit. As an aside, I don't envy the devs who have to do the cleaning - BG3 is built on the Divinity engine, and having modded DOS2 I know what a nightmare fixing conversation triggers is.
Again, thanks, that's one research item taken care of
